Lithographic projection apparatus, device manufacturing method and device manufactured thereby

ABSTRACT

In-situ cleaning of optical components for use in a lithographic projection apparatus can be carried out by irradiating a space within the apparatus containing the optical component with UV or EUV radiation having a wavelength of less than 250 nm, in the presence of molecular oxygen. Generally, the space will be purged with an ozoneless purge gas which contains a small amount of molecular oxygen in addition to the usual purge gas composition. The technique can also be used in an evacuated space by introducing a low pressure of molecular oxygen into the space.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to lithographicprojection apparatus and more particularly to lithographic projectionapparatus including a purge gas system.

[0003] 2. Background of the Related Art

[0004] A typical lithographic apparatus as described herein includes aradiation system for supplying a projection beam of electromagneticradiation having a wavelength of 250 nm or less, a support structure forsupporting patterning structure, the patterning structure serving topattern the projection beam according to a desired pattern, a substratetable for holding a substrate, and a projection system for projectingthe patterned beam onto a target portion of the substrate.

[0005] The term “patterning structure” as here employed should bebroadly interpreted as referring to means that can be used to endow anincoming radiation beam with a patterned cross-section, corresponding toa pattern that is to be created in a target portion of the substrate;the term “light valve” can also be used in this context. Generally, thesaid pattern will correspond to a particular functional layer in adevice being created in the target portion, such as an integratedcircuit or other device (see below). Examples of such patterningstructure include:

[0006] A mask. The concept of a mask is well known in lithography, andit includes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support structurewill generally be a mask table, which ensures that the mask can be heldat a desired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired.

[0007] A programmable mirror array. An example of such a device is amatrix-addressable surface having a viscoelastic control layer and areflective surface. The basic principle behind such an apparatus is that(for example) addressed areas of the reflective surface reflect incidentlight as diffracted light, whereas unaddressed areas reflect incidentlight as undiffracted light. Using an appropriate filter, the saidundiffracted light can be filtered out of the reflected beam, leavingonly the diffracted light behind; in this manner, the beam becomespatterned according to the addressing pattern of the matrix-addressablesurface. The required matrix addressing can be performed using suitableelectronic means. More information on such mirror arrays can be gleaned,for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193,which are incorporated herein by reference. In the case of aprogrammable mirror array, the said support structure may be embodied asa frame or table, for example, which may be fixed or movable asrequired.

[0008] A programmable LCD array. An example of such a construction isgiven in U.S. Pat. No. 5,229,872, which is incorporated herein byreference. As above, the support structure in this case may be embodiedas a frame or table, for example, which may be fixed or movable asrequired.

[0009] For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table; however, the general principles discussed in such instancesshould be seen in the broader context of the patterning structure ashereabove set forth.

[0010] Lithographic projection apparatus can be used, for example, inthe manufacture of integrated circuits (ICs). In such a case, thepatterning structure may generate a circuit pattern corresponding to anindividual layer of the IC, and this pattern can be imaged onto a targetportion (e.g. comprising one or more dies) on a substrate (siliconwafer) that has been coated with a layer of radiation-sensitive material(resist). In general, a single wafer will contain a whole network ofadjacent target portions that are successively irradiated via theprojection system, one at a time. In current apparatus, employingpatterning by a mask on a mask table, a distinction can be made betweentwo different types of machine. In one type of lithographic projectionapparatus, each target portion is irradiated by exposing the entire maskpattern onto the target portion at once; such an apparatus is commonlyreferred to as a wafer stepper. In an alternative apparatus—commonlyreferred to as a step-and-scan apparatus—each target portion isirradiated by progressively scanning the task pattern under theprojection beam in a given reference direction (the “scanning”direction) while synchronously scanning the substrate table parallel oranti-parallel to this direction; since, in general, the projectionsystem will have a magnification factor M (generally<1), the speed V atwhich the substrate table is scanned will be a factor M times that atwhich the mask table is scanned. More information with regard tolithographic devices as here described can be gleaned, for example, fromU.S. Pat. No. 6,046,792, incorporated herein by reference.

[0011] In a manufacturing process using a lithographic projectionapparatus, a pattern (e.g. in a mask) is imaged onto a substrate that isat least partially covered by a layer of radiation-sensitive material(resist). Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding each processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

[0012] For the sake of simplicity, the projection system may hereinafterbe referred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the protection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Twin stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO98/40791, both incorporated herein by reference.

[0013] To reduce the size of features that can be imaged using alithographic projection apparatus, it is desirable to reduce thewavelength of the illumination radiation. Ultraviolet wavelengths ofless than 200 nm are therefore currently contemplated, for example 193nm, 157 nm or 126 nm. Also contemplated are extreme ultraviolet (EUV)wavelengths of less than 50 nm, for example 13.5 nm. Suitable sources ofUV radiation include Hg lamps and excimer lasers. EUV sourcescontemplated include laser-produced plasma sources, discharge sourcesand undulators or wigglers provided around the path of an electron beamin a storage ring or synchrotron.

[0014] In the case of EUV radiation, the projection system willgenerally consist of an array of mirrors, and the mask will bereflective; see, for example, the apparatus discussed in WO 99/57596,incorporated herein by reference.

[0015] Apparatus which operate at such low wavelengths are significantlymore sensitive to the presence of contaminant particles than thoseoperating at higher wavelengths. Contaminant particles such ashydrocarbon molecules and water vapor may be introduced into the systemfrom external sources, or they may be generated within the lithographicapparatus itself. For example the contaminant particles may include thedebris and by-products that are liberated from the substrate, forexample by an EUV radiation beam, or molecules produced throughevaporation of plastics, adhesives and lubricants used in the apparatus.

[0016] These contaminants tend to adsorb to optical components in thesystem, and cause a loss in transmission of the radiation beam. Whenusing, for example, 157 nm radiation, a loss in transmission of about 1%is observed when only one or a few monolayers of contaminant particlesform on each optical surface. Such a loss in transmission isunacceptably high. Further, the uniformity requirement on the projectionbeam intensity for such systems is generally less than 0.2%. Localizedcontamination on optical components can cause this requirement not to bemet.

[0017] Previous methods for cleaning optical components include, forexample, the use of ozone as a cleaning material. However, ozone is avery unstable material and degrades only a few hours after itsformation. If ozone is to be used to clean the optical surfaces, it istherefore necessary to produce it either in situ, or immediately beforecleaning. An ozonizer may, for example, be used for this purpose.However, the extra step of producing the ozone itself is highlyinconvenient and an alternative cleaning method is desired which relieson more stable cleaning materials.

[0018] The use of more stable molecular oxygen in combination with UVradiation for cleaning purposes was contemplated by Bloomstein et al.(T. M. Bloomstein, M. Rothschild, V. Liberman, D. Hardy, N. N. EfremovJr. and S. T. Palmacci, SPIE (Optical Microlithography XIII, Ed. C. J.Progler), Vol. 4000 (2000), 1537-1545). According to Bloomstein et al.practical levels of oxygen are restricted to the range of 10 to 1000 ppmdue to absorption of 157 nm radiation.

SUMMARY OF THE INVENTION

[0019] One aspect of the invention includes providing a lithographicprojection apparatus with an efficient and improved cleaning of one ormore optical components.

[0020] This and other aspects are achieved according to the invention ina lithographic apparatus as specified in the opening paragraph, whereinthe apparatus further comprises a gas supply for supplying a purge gasto a space in said apparatus, said space containing an optical componentpositioned to interact with the projection beam, and wherein said purgegas comprises molecular oxygen at a partial pressure of from 1×10⁻⁴ Pato 1 Pa.

[0021] The inventors have found that the cleaning of optical componentsin a lithographic projection apparatus can be carried out by addition ofrelatively low partial pressures of stable molecular oxygen to a purgegas which is fed to spaces through which the projection beam travels. Asmolecular oxygen itself is not effective as cleaning agent, it is usedin combination with UV radiation. The UV radiation cracks oxygen toproduce oxygen radicals, which are highly effective cleaning agents.With the said low concentrations of cleaning agent in the purge gas, theoptical components can be cleaned while projecting a mask pattern onto atarget portion with acceptable transmission loss due to absorption of UVradiation by oxygen.

[0022] After cleaning according to the invention, the transmission orreflection of the radiation beam is increased and the uniformity mayalso be improved. The invention therefore provides a highly effectivemethod of cleaning optical components in lithographic projectionapparatus. It avoids the use of unstable materials such as ozone. Aboveall, it prevents very time-consuming dismounting of optical components(e.g. lens elements) out of the lithographic projection apparatus inorder to clean the component in a separate cleaning unit.

[0023] According to a further aspect of the present invention there isprovided a device manufacturing method comprising providing a substratethat is at least partially covered by a layer of radiation-sensitivematerial providing a projection beam of electromagnetic radiation havinga wavelength of 250 nm or less using patterning structure to endow theprojection beam with a pattern in its cross-section projecting thepatterned beam of radiation onto a target portion of the layer ofradiation-sensitive material, and cleaning an optical component for usein the apparatus by irradiating a space containing said opticalcomponent which is positioned to interact with the projection beam withthe said projection beam and supplying to said space a purge gascomprising molecular oxygen at a total partial pressure of from 1×10⁻⁴Pa to 1 Pa.

[0024] Although specific reference may be made in this text to the useof the apparatus according to the invention in the manufacture of ICs,it should be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion”, respectively.

[0025] In the present document, the terms “radiation” and “beam” areused to encompass all types of electromagnetic radiation, includingultraviolet (UV) radiation (e.g. with a wavelength of 365, 248, 193, 157or 126 nm) and extreme ultra-violet (EUV or XUV) radiation (e.g. havinga wavelength in the range 5-20 nm such as 12.5 nm) or soft x-rays, aswell as particle beams, such as ion beams or electron beams.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The invention and its attendant advantages will be furtherdescribed below with reference to exemplary embodiments and theaccompanying schematic drawings, in which:

[0027]FIG. 1 depicts a lithographic projection apparatus according tothe invention;

[0028]FIG. 2 depicts a part of the illumination system of an embodimentof the invention; and

[0029]FIG. 3 depicts a part of the illumination system of a furtherembodiment of the invention.

[0030] In the drawings, like parts are identified by like references.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0031]FIG. 1 schematically depicts a lithographic projection apparatusaccording to the invention. The apparatus comprises:

[0032] a radiation system LA, IL for supplying a projection beam PB ofUV or EUV radiation;

[0033] a first object table (mask table) MT for holding a mask MA (e.g.a reticle), and connected to first positioning means for accuratelypositioning the mask with respect to item PL;

[0034] a second object table (substrate or wafer table) WT for holding asubstrate W (e.g. a resist-coated silicon wafer), and connected tosecond positioning means for accurately positioning the substrate withrespect to item PL;

[0035] a projection system (“lens”) PL (e.g. a mirror group) for imagingan irradiated portion of the mask MA onto an exposure area C of asubstrate W held on the substrate table WT.

[0036] As here depicted, the apparatus is of a reflective type (i.e. hasa reflective mask). However, in general, it may also be of atransmissive type, for example.

[0037] The radiation system may include a source LA (e.g. an Hg lamp, anexcimer laser, a laser-produced plasma source, a discharge plasma sourceor an undulator or wiggler provided around the path of an electron beamin a storage ring or synchrotron) which produces a beam of UV or EUVradiation. This beam is caused to traverse various optical componentscomprised in the illumination system IL—e.g. beam shaping optics, anintegrator and a condenser—also included in the radiation system so thatthe resultant beam PB has a desired shape and intensity distribution inits cross-section.

[0038] The beam PB subsequently intercepts the mask MA which is held ona mask table MT. Having been selectively reflected by the mask MA, thebeam PB traverses the lens PL, which focuses the beam PB onto anexposure area C of the substrate W. With the aid of the interferometricdisplacement measuring means IF, the substrate table WT can be movedaccurately by the second positioning means, e.g. so as to positiondifferent exposure areas C in the path of the beam PB. Similarly, thefirst positioning means can be used to accurately position the mask MAwith respect to the path of the beam PB. In general, movement of theobject tables MT, WT will be realized with the aid of a long-strokemodule (course positioning) and a short-stroke module (finepositioning), which are not explicitly depicted in FIG. 1. In the caseof a waferstepper (as opposed to a step-and-scan apparatus) the masktable may be connected only to a short-stroke positioning device, tomake fine adjustments in mask orientation and position, or it may simplybe fixed.

[0039] The depicted apparatus can be used in two different modes:

[0040] In step-and-repeat (step) mode, the mask table MT is keptessentially stationary, and an entire mask image is projected at once(i.e. a single “flash”) onto an exposure area C. The substrate table WTis then shifted in the X and/or Y directions so that a differentexposure area C can be irradiated by the beam PB;

[0041] In step-and-scan (scan) mode, essentially the same scenarioapplies, except that a given exposure area C is not exposed in a single“flash”. Instead, the mask table MT is movable in a given direction (theso-called “scan direction”, e.g. the Y direction) with a speed v, sothat the projection beam PB is caused to scan over a mask image;concurrently, the substrate table WT is moved in the same or oppositedirection at a speed V=Mv, in which M is the magnification of the lensPL (typically, M=¼ or ⅕). In this manner, a relatively large exposurearea C can be exposed, without having to compromise on resolution.

[0042] In an embodiment of the present invention, the optical componentto be cleaned is an optical component within the illumination system.However, the present invention may be used to remove contaminants fromany optical component in the system, for example the mask or the opticalcomponents contained within the projection system. The present inventioncan be applied to one or several optical components eithersimultaneously or separately.

[0043]FIG. 2 shows a part of the illumination system of a specificembodiment of the invention in more detail. A space 2 within theillumination system, and containing an optical component 3, is suppliedwith a purge gas from purge gas supply 4, which may be a pressurizedcontainer containing the purge gas in gaseous or liquid form. The purgegas comprising molecular oxygen is supplied to the space 2 via inlet 5,which may comprise a valve. Space 2 now containing oxygen is thenirradiated with UV or EUV radiation, which is produced by the source LA.In this embodiment, the irradiation step is carried out at the same timeas exposure, i.e. the projection beam PB is used to crack oxygen.

[0044] Molecular oxygen within the space, when irradiated with UV or EUVradiation having a wavelength of about 250 nm or less, are cracked,forming oxygen radicals. The oxygen radicals formed act as highlyeffective cleaning agents and remove hydrocarbons and other contaminantparticles from the surface of the optical component.

[0045] In one embodiment of the present invention the space containingthe optical component to be cleaned is purged with a substantially inertgas. In this case molecular oxygen is present in a small amount in thepurge gas. The purge gas may comprise any gaseous composition which issuitable for use in a lithographic apparatus, together with oxygen.Typical purge gases comprise one or a mixture of inert gases such asnoble gases or nitrogen, together with molecular oxygen. The inventorshave found that particularly useful inert gases are argon, helium andnitrogen, for example ultra-pure nitrogen.

[0046] In particular embodiments of the present invention the purge gascompositions consist only of one or more inert gases and oxygen. In suchembodiments, it may be advantageous to remove other contaminants fromthe gas. Typically, a purifier is used to remove hydrocarbons from thepurge gas. It is possible to use a purifier in the present inventionwhich removes most hydrocarbons but does not affect the presence ofoxygen.

[0047] The total amount of molecular oxygen present in the purge gas istypically from about 1 ppb to about 10 ppm by volume. In an atmosphericenvironment, these amounts are equal to partial pressures of 1×10⁻⁴ Paand 1 Pa, respectively. If the amount of oxygen is less than about 1 ppbby volume, the amount of contaminant which is removed from the opticalcomponent may be insufficient, unless cleaning is carried out for aperiod of several hours, which is in itself undesirable. Further,concentrations of below about 1 ppb are very difficult to detect.

[0048] Alternatively, if the concentration of oxygen is above about 10ppm by volume, the absorption of the projection beam by molecular oxygenis generally so high that the transmission is decreased below anacceptable level. The level of transmission loss due to this absorptionof the projection beam depends on the path length of the optical systemto be cleaned. For example, the beam delivery system in general has amuch longer path length than the illumination system and a decrease intransmission of 10% due to UV-absorption in the beam delivery system mayequate to a decrease of around only 1% in the illumination system, giventhe same concentration of molecular oxygen. Therefore, while aconcentration of around 1 ppm may be acceptable in an illuminationsystem, systems with a longer path length may require lowerconcentrations such as 300 or 400 ppb.

[0049] In a variation of the first embodiment of the present invention,the space containing the optical component to be cleaned is evacuated.In this embodiment, the oxygen-containing species or mixture of oxygenis preferably substantially the only component(s) of the purge gas. Thepurge gas is introduced into the space at a low partial pressure. Thepressure of molecular oxygen in the space must be sufficiently high thatcontaminants can be effectively cleaned from the optical componentwithin a reasonable time, but sufficiently low that the transmission ofthe projection beam is not reduced below an acceptable level. Typically,the total partial pressure of oxygen present is from about 1×10⁻⁴ Pa toabout 1 Pa. If the pressure is below about 1×10⁻⁴ Pa, cleaning must becarried out for several hours in order to remove a sufficient amount ofcontaminant. Conversely, if the pressure is above about 1 Pa, absorptionof the (E)UV radiation by molecular oxygen is high, causing anunacceptable loss in transmission. As described above, the maximumacceptable amount of oxygen used may vary depending on the path lengthof the system to be cleaned.

[0050] If desired, the degree of contamination may be monitored usingsensor 6. Sensor 6 acts by measuring the reflectance or transmission of(E)UV radiation by the optical component to be cleaned. As is depictedin FIG. 2, the optical component may be reflective, and the sensor willtherefore measure the reflectance of the (E)UV radiation. However, ifthe optical component is of a transmissive type, the sensor will bepositioned such that it measures the degree of transmission through theoptical component.

[0051] The degree of absorption of (E)UV radiation can be used toindicate the degree of coverage of the optical component withcontaminants. In this embodiment, the system will generally be purged ofall (E)UV absorbing agents except molecular oxygen, whose concentrationis known and is preferably kept constant. Therefore, any (E)UVabsorption observed, aside from that which can be attributed to oxygenpresent, is due to the presence of contaminants. The sensor can, in thisway, be used to monitor the level of contamination, and any changes tothe level of contamination, of the optical system.

[0052] The sensor may be employed before and/or after cleaning toindicate whether the optical component in question is sufficiently cleanfor exposure to take place, or whether further cleaning is required.Regular use of this detection process may be desirable so that it can bedetermined when an optical component requires cleaning. The sensor mayalso be used during the cleaning process. Cleaning is carried out asdescribed above, and while irradiation is taking place, the absorptionof said radiation is monitored using sensor 6. When the sensor indicatesthat the absorption level has dropped below a sufficient level, and thusthe contamination level of the optical component is acceptable, thecleaning process may be stopped.

[0053]FIG. 3 depicts a third embodiment of the invention, which is thesame as the second embodiment except as described below. In thisembodiment a further source of UV or EUV radiation 7 is provided. Source7 provides radiation having a wavelength of 250 nm or less. Suitablesources of such radiation are the same as those described above withreference to source LA.

[0054] In this embodiment, the optical component 3 is irradiated byeither EUV or UV radiation having wavelengths shorter than 250 nm, whilesimultaneously projecting the patterned beam of EUV radiation.Preferably, UV radiation is used, which is capable of selectivelydissociating molecular oxygen more profoundly than EUV radiation. Forexample in the case of oxygen, UV radiation having wavelength of about157 nm is preferably used. In this way, relatively low concentrations ofoxygen in the purge gas can be employed to ensure relatively lowabsorption of EUV radiation by the cleaning agent. Consequently, theoptical component 3 can be cleaned, while exposing a wafer, withacceptable transmission loss.

[0055] It is further contemplated to irradiate the optical component 3located in space 2 using the UV or EUV radiation supplied from source 7,either before or after exposure by the projection beam PB. Preferably,irradiation is carried out before exposure, thus providing a cleanedoptical component which will improve the transmission and uniformitylevels during exposure. In this embodiment, the radiation provided bysource 7 is depicted as being directed at optical component 3. However,it is also possible to direct the radiation other than directly at theoptical component, for example across the optical component.

[0056] If desired, sensor 6 may be used to monitor the level ofcontamination as described above.

[0057] In the embodiments described above, a mask or reticle isdescribed, which may also comprise a pellicle. In a space between themask and the pellicle, a purge gas comprising molecular oxygen can besupplied in order to remove contaminants from said space according tothe above-described cleaning process.

[0058] While we have described above specific embodiments of theinvention it will be appreciated that the invention may be practicedotherwise than described. The description is not intended to limit theinvention.

1. A lithographic projection apparatus comprising: a radiation system tosupply a projection beam of electromagnetic radiation having awavelength of 250 nm or less; a support structure adapted to supportpatterning structure which can be used to pattern the projection beamaccording to a desired pattern; a substrate table to hold a substrate; aprojection system to project the patterned beam onto a target portion ofthe substrate; and a gas supply to supply a purge gas to a space in saidapparatus, said space containing an optical component positioned tointeract with the projection beam, wherein said purge gas comprisesmolecular oxygen at a total partial pressure of from 1×10⁻⁴ Pa to 1 Pa.2. An apparatus according to claim 1, wherein said purge gas furthercomprises an inert gas selected from the group comprising helium, argon,nitrogen and mixtures thereof, and wherein the total amount of molecularoxygen present in said purge gas is from 1 ppb to 10 ppm by volume. 3.An apparatus according to claim 1, wherein said space is substantiallyevacuated.
 4. An apparatus according to claim 1, which apparatus furthercomprises a further supply of electromagnetic radiation having awavelength of 250 nm or less and arranged to supply such radiation ontosaid optical component.
 5. A device manufacturing method comprising:projecting a patterned beam of radiation having a wavelength of 250 nmor less onto a target portion of a layer of radiation-sensitive materialon a substrate, and irradiating a space containing an optical componentof a lithographic projection apparatus which is positioned to interactwith the projection beam with the projection beam and supplying to saidspace a purge gas comprising molecular oxygen at a total partialpressure of from 1×10⁻⁴ Pa to 1 Pa.
 6. A method according to claim 5,wherein the purge gas further comprises one of an inert gas, preferablyhelium, argon, nitrogen and mixtures thereof, and wherein the totalamount of molecular oxygen present in said purge gas is from 1 ppb to 10ppm by volume.
 7. A method according to claim 5, wherein said space issubstantially evacuated.
 8. A method according to claim 5, furthercomprising: supplying a further beam of electromagnetic radiation havinga wavelength of 250 nm or less, and irradiating said optical componentwith said further beam of electromagnetic radiation, while projectingsaid patterned beam of radiation onto said target portion.